Added doc for LinearPolar Transform

Tickets: #1729 and #3654

LogPolar is documented and has samples for both C ("samples/c/polar_transforms.c") and python ("samples/python/logpolar.py"). 

LinearPolar is not documented. It has a C sample ("samples/c/polar_transforms.c"). There seems to be no equivalent function as well as a sample in python.

However, OpenCV 3.x has cv2.linearpolar ("samples/python/logpolar.py").